2:25 amOff
Gel That Flash
When you want to harmonize the color of your flash with the ambient light, you first have to know which color the ambient light is, obviously. You might try to go at it by eye, or even by tradition, but you might not get the result you expected (for example, fluorescent light used to be very green, but this is not necessarily a reliable presumption anymore, since they now come in many colors, with arbitrary names like "cool white").
Since we don't walk around with colorimeters or computers to analyze our images (and because we just don't have the time to fool around anyway), there is an easy trick to reliably figure out which color the ambient light actually is. Your camera has an RGB histogram — all you need to do is to fill the frame with a neutral reference and you'll know!
Remember that since you only want to read the ambient light for this test, you need to turn off your flash. Also, it is important to use the "daylight" (neutral) white balance to do this test, otherwise you will not get a true assessment of the color (you will get something that was corrected in one way or another). The neutral reference could be a grey card (ideally), but it could also just be a white wall or piece of paper — as long as it fills the frame and is "close enough" to neutral you'll be alright, since you won't be able to surgically match your flash to that color anyway (there's only so much you can do with a couple of gels!)
I tested this in my living room, where the lamps use "compact fluorescent" bulbs — those are hard to guess. When the image appears on your LCD, display the RGB histogram, and you will see the color dominance right there:
In this case, we can see that there is far more red and green than blue. We can also see that although this is fluorescent, this is definitely not just green. Remember that with light, red+green=yellow! So in this case we have some kind of orange, because the red component is a bit stronger than the green.
Now all you have to do is pick gels that will add the same cast to your flash. If I had to act quickly, I would pick a strong orange and I know that I would get a pretty decent result. With just this one quick test, I am certain not to make a huge mistake and use a color that is way off.
To illustrate the effect the different gels produce on your flash, you can do a similar test: shoot a neutral reference, in "daylight" white balance, but this time with only the flash exposure (use a fast shutter speed and low ISO to remove any ambient light).
Using a strong green gel, we get this result:
Definitely not! Notice how the histogram doesn't look at all like the one I had with the ambient light only. Let's try a strong orange gel:
Well, this is much better. I could certainly shoot knowing that the color of the ambient light and the color of my flash are "close enough". If you want to match the ambient light even more closely, it's possible — at some point you'll get something like this:
A-ha! Well this is very, very close. But I had to use three different gels (from what I had in my kit), so this is getting a bit crazy. Also remember that each gel you add in front of your flash cuts some light — the more gels you use, the less powerful your flash becomes.
In practice, you might only have 2 or 3 degrees of orange and green in a basic kit. That's fine — most discrepancies can be removed with just these, even if they are not "perfect". The more you do this, the more you will be able to pick the appropriate gel (or combination of gels), since you will know what histogram they produce.